1. Field of the Invention
This invention relates generally to spinal fusion implants, and more particularly to spinal fusion implants for insertion from the side of a patient (translateral) across the transverse width of the spine and between two adjacent vertebrae.
2. Description of the Related Art
In the past, spinal fusion implants have been inserted only from either an anterior or posterior direction, from the front or the back of the patient. Such implants are well known in the art and may have cylindrical, rectangular and other shapes. In the past, Cloward, Wilterberger, Crock, Viche, Bagby, Brantigan, and others have taught various methods involving the drilling of holes across the disc space between two adjacent vertebrae of the spine for the purpose of causing an interbody spinal fusion. Cloward taught placing a dowel of bone within that drilled hole for the purpose of bridging the defect and to be incorporated into the fusion. Viche taught the threading of that bone dowel. Bagby taught the placing of the bone graft into a metal bucket otherwise smooth on its surface, except for rows of radially placed holes communicative to the interior of the basket and to the bone graft. The Bagby device was disclosed as capable of being used in a horse. Brantigan taught the use of inert blocks preferably made of metal and having that metal at its external surface imitate the porosity of bone. Brantigan theorized that the bone dowel could be replaced entirely with a metal plug that, while not itself active in the fusion, would nevertheless serve to support the vertebrae from within the disc space while allowing fusion to occur around it.
U.S. Pat. No. 3,844,601 issued to Ma et al. on Nov. 19, 1974, teaches a method and instrumentation for preparing rectangular spaces across the disc space into the adjacent vertebrae and for preparing a rectangular graft of the bone itself that is inserted in the rectangular spaces.
U.S. Pat. No. 4,743,256 issued to Brantigan on May 10, 1988 teaches the use of an inert artificial spacer in the shape of a rectangle in place of using a rectangular bone graft as taught by Ma et al.
U.S. Pat. No. 4,878,915 issued to Brantigan on Nov. 7, 1989, teaches the use of fully cylindrical inert implants for use in interbody spinal fusion. Such implants do not participate in the bone fusion process but act as inert spacers and allow for the growth of bone to the outer surfaces of the implants.
U.S. Pat. No. 4,834,757 issued to Brantigan on May 30, 1989, teaches a rectangular shaped, hollow spinal fusion implant for use in lieu of a rectangular bone graft or Brantigan's earlier artificial inert spacer.
However, all of the prior implants have been inserted from either the front or the back of the patient. As a result, the spinal fusion implants of the past were necessarily limited in size to the dimensions of the vertebrae relative to the direction in which the implants were inserted. For example, the maximum possible length for an implant that is inserted from either the front or the back of the patient is limited to the depth of the vertebrae, the depth of a vertebrae being the dimension of the vertebrae measured from the anterior end to the posterior end of the vertebrae. It was not previously possible to insert an implant that had a length that was greater than the depth of the vertebrae from front to back as such an implant would protrude from either the anterior or posterior aspect of the spine resulting in great harm to the patient.
In U.S. Pat. No. 5,015,247 to Michelson, a cylindrical threaded implant is described for insertion across the disc space between two adjacent vertebrae. Such an implant was disclosed as being inserted either from the front of the patient or from the back and has a diameter larger than the disc space so that it engages both of the adjacent vertebrae.
The maximum diameter possible with a cylindrical implant that is inserted from the front or the back of the patient is limited by at least two factors. The first factor limiting the diameter of a cylindrical implant is realized when an attempt is made to use a single, centrally placed implant from either the front or the back of the patient. Such an implant must be large enough to occupy a sufficient portion of the transverse width of the disc space to promote firm stability. The use of an implant that is placed in the disc space to stabilize the two adjacent vertebrae requires that the vertebrae be stable when the implant is in place, otherwise there will not be any bone bridging between the implant and the vertebrae. If a single implant is used in the center of the disc space, inherent instability is created, as the vertebrae are generally free to rock back and forth over the implant which serves as a fulcrum. However, to achieve the required stability, it would be necessary to use the widest possible implant and the excursion of such a large single implant into the adjacent vertebrae would be so severe that the two vertebrae would be virtually cut in half.
The second factor which limits the diameter size of a cylindrical implant is in the situation where two cylindrical implants are implanted from either the front or the back of the patient and placed side-by-side across a disc space and into the two adjacent vertebrae in an attempt to gain stability while avoiding the problems of the single implant. Such implants require a diameter that is sufficiently large to penetrate into and significantly engage each of the adjacent vertebrae yet the diameter may not be so large that it is no longer possible to place two such implants side-by-side and to still have them contained within the transverse width of the spine.
The use of multiple implants requires that the implants be small enough so as to fit into the same limited spinal width. These implants being of smaller diameter as limited by the need to place more than one within the width of the spine then penetrate only minimally into the depth of the vertebral bone.
Also, the insertion of multiple implants requires multiple procedures, essentially a duplication of any procedure done on one side of the center line must also be performed on the other side of the center line.
Therefore, there exists a need for a spinal fusion implant that is inserted from the translateral approach to the spine that is capable of stabilizing the vertebrae adjacent to such an implant in order to permit bone bridging between the vertebrae and the implant to ultimately achieve fusion of the adjacent vertebrae.